Hitherto, organic electroluminescent (hereinafter abbreviated as “organic EL”) elements are known as light-emitting devices. Generally, organic EL elements have a multi-layer structure including a positive electrode, a negative electrode, and charge injection layers (i.e., a hole injection layer and an electron injection layer) and a light-emitting layer, which intervene between the positive electrode and the negative electrode. In operation, injected electrons recombine with holes in the light-emitting layer, to thereby emit light.
In such a light-emitting layer and a charge injection layer, a charge-transporting thin film formed of an organic compound is employed. In particular, a hole injection layer disposed between the positive electrode and the light-emitting layer is involved in charge transfer to/from the positive electrode and the light-emitting layer, and thus plays an important role in attaining low driving voltage and high luminance of an organic EL element. Accordingly, various studies are now conducted on the material of a charge-transporting thin film for use in an organic EL element, in order to enhance characteristics of the organic EL element.
Meanwhile, the method of forming a charge-transporting thin film generally includes a dry process (typically, vacuum vapor deposition) and a wet process (typically, spin coating). Of these, a wet process is more advantageous, in that a large-area thin film with high flatness can be effectively formed.
Under such circumstances, there has been proposed a charge-transporting compound having a structure in which the two nitrogen atoms forming the indolocarbazole ring are substituted with an arylene group or a heteroarylene group (see, for example, Patent Document 1). There has also been proposed a charge-transporting compound having a structure in which the two nitrogen atoms forming the indolocarbazole ring are substituted with a polymerizable group (see, for example, Patent Document 2). There has also been proposed a charge-transporting compound having a structure in which the two nitrogen atoms forming the indolocarbazole ring are directly bonded to a main chain (see, for example, Patent Document 3). Some documents disclose synthesis of indolocarbazole compounds (see, for example, Non-Patent Document 1).